1. Field of the Invention
The present invention relates to an optical waveguide circuit such as an arrayed waveguide grating type optical multiplexer/demultiplexer used in optical communications or the like.
2. Description of the Related Art
In recent years, optical wavelength division multiplexing communications has been actively researched and developed in the hope of increasing exponentially data transmission capacity in optical communications, and it is beginning to be put into practice. Optical wavelength division multiplexing communications is to transmit data by putting, for example, a plurality of light beams having different wavelengths through wavelength division multiplexing. In such an optical wavelength division multiplexing communications system, the transmitted plural light beams with different wavelengths have to be picked separately on the basis of the wavelength by the receiver of the light beams. Therefore, a light transmissive element that transmits only a light beam having a predetermined wavelength, or like other elements, is indispensable for the system.
An example of the light transmissive element is an arrayed waveguide grating (AWG) shown in FIG. 1. The arrayed waveguide grating has, on a substrate 11 of silicon or the like, an optical waveguide forming region structure as shown in FIG. 1. The optical waveguide structure of the arrayed waveguide is composed of: one or more optical input waveguides 12 arranged side by side; a first slab waveguide 13 connected to the exit ends of the one or more optical input waveguides 12; an arrayed waveguide 14 composed of plural channel waveguides 14a arranged side by side connected to the exit end of the first slab waveguide; a second slab waveguide 15 connected to the exit end of the arrayed waveguide 14; and a plurality of optical output waveguide 16 arranged side by side and, connected to the exit end of the second slab waveguide. The arrayed waveguide 14 propagates light that is outputted from the first slab waveguide 13, and is composed of a plurality of channel waveguides 14a arranged side by side. Lengths of adjacent channel waveguides are different each other with the difference preset. The number of optical output waveguides 16 is determined, for example, in accordance with how many light beams having wavelengths different from one another are to be created as a result of demultiplexing or multiplexing of signal light by the arrayed waveguide grating. The channel waveguides constituting the arrayed waveguide are usually provided in a large number 100 for example. However, FIG. 1 is simplified and the number of the channel waveguides, the optical output waveguide 16, and the optical input waveguides 12 in FIG. 1 does not exactly reflect the actual number therof.
The optical input waveguides 12 are connected to, for example, transmission side optical fibers so that light having undergone the wavelength division maltiplexing is introduced to the optical input waveguides. The light having traveled through the optical input waveguide and been introduced to the first slab waveguide, is diffracted by the diffraction effect thereof, and enters the arrayed waveguide to travel along the arrayed waveguide.
Having traveled through the arrayed waveguide 14, the light reaches the second slab waveguide 15, and then is condensed in the optical output waveguides 16 to be outputted therefrom. Because of the preset difference between adjacent channel waveguides 14a of the arrayed waveguide 14, the light beams after traveling through the arrayed waveguide 14 have phases different from one another. The wavefront of the traveled light is tilted in accordance with this difference and the position where the light is condensed is determined by the angle of this tilt. Therefore, the light beams having different wavelengths are condensed at positions different from one another. By forming the optical output waveguides 16 at these positions, the light beams having different wavelengths can be outputted from their respective optical output waveguides 16 that are provided for the respective wavelengths.
For instance, as shown in FIG. 1, light beams having undergone the wavelength division multiplexing and having wavelengths of xcex1, xcex2, xcex3 . . . xcexn (n is an integer equal to or larger than 2), respectively, are inputted to one of the optical input waveguides 12. The light beams are diffracted in the first slab waveguide 13, reach the arrayed waveguide 14, and travel through the arrayed waveguide 14 and the second slab waveguide 15. Then, as described above, the light beams are respectively condensed at different positions determined by their wavelengths, enter different optical output waveguides 16, travel along their respective optical output waveguides 16, and are outputted from the exit ends of the respective optical output waveguides 16. The light beams having different wavelengths are taken out through optical fibers for outputting light that are connected to the exit ends of the optical output waveguides 16.
In this arrayed waveguide grating, wavelength resolution of the grating is in proportion to the difference in length (xcex94L) among the adjacent channel waveguides 14a of the arrayed waveguide 14 that constitutes the grating. When the arrayed waveguide grating is designed to have a large xcex94L, it is possible to multiplex/demultiplex light to accomplish wavelength division multiplexing with a narrow wavelength interval, which has not been attained by other type optical multiplexer/demultiplexer of prior art. It is thus possible for the arrayed waveguide grating to have a function of multiplexing/demultiplexing a plurality of signal light beams, specifically a function of demultiplexing or multiplexing a plurality of optical signals with a wavelength interval of 1 nm or less, which is a function deemed necessary for optical wavelength division multiplexing communications of high density.
The above arrayed waveguide grating is an optical waveguide circuit in which an optical waveguide portion 10 having an under cladding, a core and an over cladding formed from silica-based glass or the like is formed on the substrate 11 of silicon or the like. The under cladding is formed on the substrate 11, the core with the above optical waveguide structure is formed thereon, and the over cladding is formed on the core to cover the same. The over cladding is formed from silica-based glass obtained by, for example, doping pure silica with a 6 mol % of B2O3 and a 6 mol % of P2O5 (SiO2xe2x80x94B2O3xe2x80x94P2O5).
FIGS. 4A to 4D illustrate a process of manufacturing the arrayed waveguide grating, and described below with reference to FIGS. 4A to 4D is a method of manufacturing the optical waveguide circuit. First, as shown in FIG. 4A, a layer for an under cladding 1b is formed on the substrate 11 and a layer for a core 2 is subsequently formed thereon. Next, the layer for the core 2 to form an optical waveguide pattern of the arrayed waveguide grating, thereby forming the core 2 with the optical waveguide structure described above as shown in FIG. 4C, by photolithography reactive ion etching method using a mask 8 as shown in FIG. 4B.
Then a layer for an over cladding 1a is formed on the core 2 so as to cover the core 2 as shown in FIG. 4D. The each layer for the under cladding, the core and the over cladding 1a is formed by flame hydrolysis deposition method and consolidating the glass particles 50 at a temperature of, for example, 1200xc2x0 C. to 1250xc2x0 C.
In the optical wavelength division multiplexing communications as above, when only a horizontally polarized wave is transmitted as signal light, a vertically polarized wave perpendicular to the horizontally polarized wave turns into a noise that degrades the transmission characteristic of the above communications. The noise causes reduction in data transmission capacity and transmission distance and, hence, fewer vertically polarized wave is better. On the other hand, when only a vertically polarized wave is transmitted as signal light in the above optical wavelength division multiplexing communications, horizontally polarized wave perpendicular to the vertically polarized wave has to be reduced as much as possible.
In other words, polarization crosstalk (i.e., extinction ratio of signal light to noise light polarized in the direction perpendicular to the polarization of the signal light) has to be as small as possible in the optical wavelength division multiplexing communications system. Specifically, a desirable polarization crosstalk is xe2x88x9220 dB or less in total for the entire optical wavelength division multiplexing communications system. The polarization crosstalk is expressed, for instance, as the following expression (1):
Polarization crosstalk=10 log(Py/Px)xe2x80x83xe2x80x83(1)
wherein horizontally polarized wave intensity Px (polarized in the direction x) is the signal light and vertically polarized wave intensity Py (polarized in the direction y) is the noise light.
Accordingly, each of the optical components used in the optical wavelength division multiplexing communications system has to have a polarization crosstalk smaller than xe2x88x9220 dB. However, in a conventional optical waveguide circuit adopted as the light transmissive element of the arrayed waveguide grating, or the like, the horizontally polarized wave and the vertically polarized wave are transmitted together if light entering the circuit has both This makes it difficult to reduce the polarization crosstalk between a polarized wave serving as signal light and a polarized wave perpendicular to the former wave.
The present invention has been made to solve the problems above, and an object of the present invention is therefore to provide an optical waveguide circuit that has a small polarization crosstalk and is suitable for optical wavelength division multiplexing communications.
In order to attain the above object, the present invention provides an optical waveguide circuit having the following construction. The optical waveguide circuit of the present invention is comprised such that:
an optical waveguide portion is formed on a substrate;
the optical waveguide portion has an under cladding, a core, and an over cladding formed from silica-based glass; and
birefringence B in the core is set so as to satisfy |B|xe2x89xa71.2xc3x9710xe2x88x924.
In the optical waveguide circuit above, when the thermal expansion coefficient of the over cladding is given as xcex1g and the thermal expansion coefficient of the substrate is given as xcex1s, xcex1g xe2x89xa6xcex1sxe2x88x924.39xc3x9710xe2x88x927 is satisfied.
According to another aspect of the present invention, an arrayed waveguide grating with the above optical waveguide circuit is provided. The arrayed waveguide grating comprises: one or more optical input waveguides arranged side by side; a first slab waveguide connected to the exit ends of the optical input waveguides; an arrayed waveguide connected to the exit end of the first slab waveguide to transmit light guided by and outputted from the first slab waveguide and composed of a plurality of channel waveguides, the channel waveguides being arranged side by side and having different lengths with the difference preset; a second slab waveguide connected to the exit end of the arrayed waveguide; and a plurality of optical output waveguides arranged side by side and connected to the exit end of the second slab waveguide.
The present inventors have turned their attention to the value of birefringence in the optical waveguide portion that constitutes the optical waveguide circuit and has the under cladding, the core and the over cladding in order to reduce the polarization crosstalk in the optical waveguide circuit of the arrayed waveguide grating or the like. To be specific, in the arrayed waveguide grating, for instance, light condensing positions for a horizontally polarized wave and a vertically polarized wave vary depending on the value of the birefringence. Therefore, the present inventors have thought that a polarized wave serving as noise (a vertically polarized wave when the signal light is a horizontally polarized wave, whereas the noise is a horizontally polarized wave when the signal light is a vertically polarized wave) can be removed by setting the birefringence to a proper value.
Then the inventors have examined for the case of the arrayed waveguide grating the relation between the birefringence in the optical waveguide portion and the polarization crosstalk. As a result of examination, it has been found that, when the arrayed waveguide grating is applied to an optical communications system composed of a polarization maintaining device and a birefringence B is set so as to satisfy |B|xe2x89xa71.2xc3x9710xe2x88x924, a polarized wave serving as noise can be removed effectively by the arrayed waveguide grating and the polarization crosstalk can be reduced to xe2x88x9220 dB or less.
According to the present invention, the birefringence B is set to a value that makes it possible to reduce the polarization crosstalk to xe2x88x9220 dB or less (|B|xe2x89xa71.2xc3x9710xe2x88x924) based on the results of examination of the relation between the value of the birefringence in the core and the polarization crosstalk. Therefore, the present invention can reduce the polarization crosstalk in the optical waveguide circuit of the arrayed waveguide grating or the like, thereby making the optical waveguide circuit suitable for optical wavelength division multiplexing communications.
Moreover, if xcex1gxe2x89xa6xcex1sxe2x88x924.39xc3x9710xe2x88x927 is satisfied when the thermal expansion coefficient of the over cladding is given as xcex1g and the thermal expansion coefficient of the substrate is given as xcex1s, the birefringence B can surely be set to the value that makes it possible to reduce the polarization crosstalk to xe2x88x9220 dB or less. The effect above can thus be exerted correctly.